This invention relates to a method for delivering beneficial liquids such as fragrances, deodorizers, sanitizers, pesticides and pest repellants at a steady rate for extended time periods using an osmotic pump and where the source of water for the osmosis typically is not continuously refreshed.
What is needed are methods for delivering liquid beneficial agents, such as fragrances, de-odorizers, sanitizers, pesticides and pest repellants in a controlled, predictable manner. Ideally, such methods would be suitable to disperse a wide variety of different beneficial liquids products which may be solutions, suspensions, or mixtures. Further needed are methods for easily controlling the rate at which the beneficial agents are released.
Many have investigated delivering liquids using osmotic engines. In general, an osmagent is contained in a variable volume container that in part includes a semipermeable membrane and also communicates with a container containing a beneficial agent through a flexible diaphragm, piston or such. Upon activation, the semipermeable membrane is exposed to a source of water. Water flows through the semipermeable membrane into the osmagent container, expanding the volume, which in turn forces the beneficial agent to be expelled. In some cases the devices are implanted into the body of an animal or human where the body is the source of water. In other cases, the water is supplied from a reservoir contained in the device.
Herbig et al. in U.S. Pat. No. 5,798,119 disclosed a device used for delivering fluids such as fragrances and insecticides. They used a hydrophobic microporous separator to separate an osmagent from liquid water. Water vapor passes through the hydrophobic membrane from the liquid water to the osmagent, increasing the volume where the osmagent is located. The volume increase drives the delivery of the beneficial agent. A disadvantage of this approach is that water vapor pressure is very temperature dependant. For example, water vapor pressure is 20× higher at 50 C compared to 0 C. Looking at a narrower temperature range. The vapor pressure at 10 C is 56% lower than at 23 C and at 44 C the vapor pressure is 326% that of 23 C. Thus temperature variations will have a very large impact on the dispense rate with this type of system which is very undesirable in most cases.
Faste in U.S. Pat. No. 4,898,582 and Atahyde et al. in U.S. Pat. No. 5,672,167 disclosed drug infusion devices using osmosis where the water was contained within the device. These inventors disclosed systems utilizing cellulose ester or cellulose ether membranes such as cellulose acetate as the semipermeable membrane between the osmagent and the water source. An advantage of these membranes over the hydrophobic membranes disclosed by Herbig et al. is the fact that liquid water diffuses through the semipermeable membranes rather than water vapor. This significantly reduces the temperature sensitivity of the osmosis since the concentration of water is substantially unchanged over a temperature range as opposed to widely varying water vapor pressure. A disadvantage of these membranes is that while they are substantially semipermeable, they still have permeability to many potential osmagents. As a result, the osmagent can permeate into the water container as well as water diffusing into the osmagent container. While the diffusion of osmagent is small, the effect over time can be very large when the volume of water contained is near the same amount of liquid to be dispensed and especially if the time scale of delivery is long. As osmagent diffuses into the water container, the driving force for diffusion of water across the semipermeable membrane is reduced and the delivery rate declines over time.
Several inventors such as Wong et al. in U.S. Pat. No. 4,874,388 and Chen et al. in U.S. Pat. No. 6,923,800 disclose osmotically driven devices where the devices are implanted into the body of animal or man where the water is supplied by the body and where the concentration of the water near the semipermeable membrane remains nearly the same over time due to the active nature of the body. Wong et al. describe the use of “cellulosic polymers such as cellulose acetate, ethyl cellulose, methylcellulose, cellulose acetate butyrate, cellulose acetate propionate, blends of impermeable material and hydrophilic polymer or a molecular weight water soluble enhancer to render the material semipermeable”. Chen et al. on the other hand disclosed using polyurethane materials which are somewhat permeable to water for low rate devices.
The prior art does not teach how to obtain steady fluxes of water through a semipermeable membrane where osmagent is on one side and a non continuously refreshed water source is opposite and where variation in rate due to changes in temperature are minimal.